LHC Signatures of Neutral Scalar Cascades in the $Z_3$ symmetric 3HDM

TL;DR

Problem: test the collider signatures of an extended Higgs sector beyond the SM using a $Z_3$-symmetric 3HDM with Type-Z Yukawa. Approach: perform a cut-and-count analysis of the cascade channel $pp\rightarrow A\rightarrow HZ$ with $H\to bb$ and $Z\to \ell^+\ell^-$ under alignment, exploring Regular and Medial mass hierarchies at $\sqrt{s}=14$ TeV. Key results: Medial Hierarchy yields discovery-level sensitivity at $200\ \mathrm{fb}^{-1}$, while Regular Hierarchy requires much higher luminosity (HL-LHC scale); the alignment limit suppresses $A-H_{\text{SM}}-Z$ couplings and emphasizes cascade pathways with non-SM CP-even states. Significance: demonstrates that neutral scalar cascades in the 3HDM provide a complementary, testable probe of extended Higgs sectors at the LHC and informs future searches and parameter-space explorations.

Abstract

Extending the scalar sector is one of the standard approaches to exploring scenarios beyond the Standard Model. In this work, we examine the collider phenomenology of the Three Higgs Doublet Model (3HDM) in the Type-Z or the democratic Yukawa interaction setup at the LHC. The scalar spectrum of the 3HDM includes three CP-even scalars, two CP-odd scalars, and four charged Higgs bosons. Focusing on cascade decay topologies, we investigate the collider signatures of the neutral scalars through the process $pp \rightarrow SV$, where $S$ is a neutral scalar and $V$ is a vector boson. We perform a cross-section analysis across multiple benchmark points that satisfy both theoretical and experimental constraints, considering two mass hierarchy scenarios: (i) Regular Hierarchy, where the SM-like Higgs is the lightest CP-even scalar, and (ii) Medial Hierarchy, featuring one Higgs boson lighter than the SM Higgs and one heavier. For both scenarios, we study the specific process $pp \rightarrow A \rightarrow HZ \rightarrow b \bar{b} l^+l^-$, performing a cut and count analysis at $\sqrt{s}=14$ TeV. Our results demonstrate that while the Medial Hierarchy scenario allows discovery-level sensitivity for both the CP-even and CP-odd scalars, achieving the same sensitivity in the Regular Hierarchy setup necessitates substantially higher luminosity.

Figures (7)

• Figure 1: The allowed regions for the Regular Hierarchy case with the lightest of the three CP-even scalars identified as the 125 GeV SM-like Higgs boson. The plots show how the allowed region in $m_{H_2}$ correlates with the various angles. The color coding of the plots is as follows: regions shaded in blue is allowed by the stability, perturbativity and unitarity constraints while the red region is allowed by the direct search constraints and the goodness of fit test, and also the $b \rightarrow s \gamma$ flavor constraint. The electroweak precision constraints are indicated by the green shaded region.
• Figure 2: The allowed regions for the Medial Hierarchy case with the second lightest CP-even Higgses identified as the 125 GeV SM-like Higgs boson. The plots show how the allowed region in $m_{H_1}$ correlates with the masses of the other Higgses in the theory.
• Figure 3: The x-axis shows the cross-section for the process $pp \rightarrow S Z \rightarrow b \bar{b} l^+ l^-$, while the y-axis displays the cross-section for the process $pp \rightarrow S W^+ \rightarrow b \bar{b} l^+ \nu$, where $S$ denotes a neutral scalar particle.
• Figure 4: Feynman diagram illustrating the gluon fusion production of the CP-odd scalar, with its subsequent decay to a CP-even scalar and a Z boson. The effective coupling of the $A$ to $gg$, denoted by the circle, involves both top and bottom loop contributions.
• Figure 5: The branching ratio of the cascade decay of a CP-odd scalar into a CP-even Higgs, plotted as a function of the CP-odd scalar mass. For the Regular Hierarchy (top panel), the branching ratio of both CP-odd scalar is displayed, whereas for the Medial Hierarchy (bottom panel) the corresponding branching ratio of the state $A_2$ is presented.